The Navy, for some years, has used the USQ-20(V)/CP642 computer series manufactured by Sperry Univac Corporation (Defense Systems Department) in its tactical data system. The CP642 series computers were originally designed and built with 32,000 words of core memory in each unit. The first venture into extended memory was a device called the mass data storage system. The mass data storage system had two possible modes of operation; peripheral access and direct access. Due to the desirability of extending the CP642 series computers on executable memory (a direct access mode), the peripheral access mode was not pursued to any significant extent. An adapter unit was required to interface the mass data storage system with the CP 642 computer. Memory addressing beyond the normal 32,000 words of CP 642 was accomplished by a paging scheme. The paging technique employed the basic 15-bit address within the computer which in turn was partitioned by the adapter unit outside the computer into a 3-bit page designator address containing 6-bits appended to the remaining 12-bit addresses, resulting in an 18-bit address. A major limitation with this paging scheme was that the input/output had to be carefully controlled from within the computer and new instructions were required to set and reset page register values. This approach was costly in software changes to allow effective use of the extended memory. A need then existed for extending the memory of the CP642 computer series which did not require these inordinate amounts of software changes.